1. Field of the Invention
This invention relates in general to semiconductor lasers, and in particular to a tunable laser source with integrated optical modulator.
2. Description of the Related Art
Modern day usage of optical components and lasers has made communications and data transfer more efficient and more cost effective. The use of semiconductor lasers has made the fabrication and packaging of optical sources more cost effective, as well as reducing the size of the overall device.
However, the requirements for communications and data transfer systems have also increased. Widely tunable lasers are essential components for a wide variety of wavelength-division multiplexing (WDM) and packet switching network architectures. They can be used as replacement sources in long haul dense WDM communication systems or for wavelength routing in access networks. They are also important devices for next generation phased array radar systems that use true-time delay beam steering.
In order to achieve a wide tuning range, these devices require fairly large passive tuning elements. This makes the devices four to five times larger than conventional fixed wavelength lasers. However, having this large amount of passive material in a laser cavity reduces the speed with which they can be turned on and off by direct current modulation. Moreover, the rate at which they are able to transmit data is limited, making them unsuitable for high bandwidth applications.
There are two other factors that make it difficult to use these devices to transmit data. The wavelength in a sampled grating distributed Bragg reflector (SGDBR) laser is controlled by aligning a pair of reflection peaks in two mirrors with an optical cavity mode. When a gain current is modulated over a wide range of currents, it can disturb this alignment, resulting in mode instability within the device, which is highly undesirable for data transmission. To prevent this mode instability, such devices can only be modulated over a narrow range of output powers, which introduces a significant extinction ratio penalty to their data transmission performance.
The other problem with directly modulating a laser is frequency chirp, which is the shift in the laser oscillation frequency that occurs when the output power level is changed. This is undesirable in transmission systems, since frequency chirp causes pulse spreading, which limits the maximum distance over which data can be sent over an optical fiber or other dispersive medium.
The three most successful types of widely tunable lasers are the super structure grating distributed Bragg reflector laser (SSGDBR), the grating assisted codirectional coupler with sampled grating reflector laser (GCSR), and the sampled grating DBR laser (SGDBR). All of these devices are capable of continuous tuning ranges greater than 40 nm. However, SGDBR lasers and other widely tunable designs have long active sections and fairly large optical cavities that limit their direct modulation bandwidth to between 3 and 4 GHz. This enables them to be used in OC-48 data transmission systems under direct modulation, if some wavelength chirping can be tolerated. However, this bandwidth is insufficient for use in most phased array radar systems or in OC-192 data transmission networks operating at 10 Gb/s.
In these applications, external modulators are frequently used to apply a radio frequency (RF) signal or data to the optical carrier. Even for long-haul OC-48 systems, external modulators are frequently used to minimize frequency chirp. However, external modulators add significant cost and complexity to the optical assembly which can be prohibitive in systems that require a large number of tunable lasers and modulators. For this reason, it is desirable to monolithically integrate a high speed modulator with a tunable laser on as a single semiconductor device.